Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2018

Summary of Assumptions Used in Black-Scholes Option Pricing Model

The fair values of stock options granted during the three and nine months ended September 30, 2018 and 2017 were estimated as of the dates of grant using the Black-Scholes option pricing model with the following assumptions:

	Three Months Ended September 30,		Nine Months Ended September 30,
	2018	2017	2018	2017
Risk-free interest rate	2.73% - 2.99%	1.87% - 1.97%	2.51% - 2.99%	1.85% - 2.57%
Expected volatility	82.6% - 87.6%	81.4% - 81.7%	82.6% - 92.8%	75.3% - 83.3%
Expected dividend yield	0%	0%	0%	0%
Expected life of employee options (in years)	6.25	6.25	6.25	6.25
Expected life of non-employee options    (in years)	—	—	—	10

Summary of Compensation Expense Relating to Stock Options

During the three and nine months ended September 30, 2018 and 2017, the Company recognized compensation expense in the accompanying Condensed Statements of Comprehensive Loss relating to stock options, as follows:  

	Three Months Ended September 30,				Nine Months Ended September 30,
	2018		2017		2018		2017
Research and development	$	901	$	619	$	2,401		1,784
General and administrative		918		1,475		3,358		2,736
Total stock option expense	$	1,819	$	2,094	$	5,759	$	4,520

Summary of Stock Option Activity

A summary of stock option award activity related to employees, non-employee members of the Company’s Board of Directors and non-employee consultants as of and for the nine months ended September 30, 2018 is presented below:

	Number of Shares			Weighted Average Exercise Price
Outstanding, December 31, 2017		3,492,141		$	11.75
Granted		897,500			15.12
Exercised		(377,433	)		9.64
Expired		(58,395	)		10.65
Forfeited		(173,423	)		12.12
Outstanding, September 30, 2018		3,780,390		$	12.76
Options exercisable, September 30, 2018		1,797,351

Non-employee Consultants [Member]

Summary of Compensation Expense Relating to Stock Options

Included in the table above are the following amounts of compensation expense recognized with regard to stock options that were granted to non-employee consultants, including the effect of re-measurement of the fair values of those options, as described above:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2018		2017		2018		2017
Research and development	$	68	$	9	$	189	$	156
General and administrative		13		70		192		103
Total stock option expense	$	81	$	79	$	381	$	259

Re-Measurement [Member]

Summary of Assumptions Used in Black-Scholes Option Pricing Model

As of September 30, 2018 and 2017, the Company used the Black-Scholes option valuation model with the following ranges of assumptions to re-measure the fair value of all outstanding options that had been granted to non-employee consultants during the vesting period of each tranche in accordance with ASC 505-50:  

	September 30,
	2018	2017
Risk-free interest rate	2.06% - 3.02%	1.28% - 2.28%
Expected volatility	77.4% - 80.9%	75.2% - 84.0%
Expected dividend yield	0%	0%
Expected life of non-employee options (in years)	0.26 - 8.44	0.88 - 9.44